plagiarism?

Probably because this entire argument really doesn't have a single leg to stand on. There's absolutely no plagiarism to speak of or even any real hint of it. Nothing about the marathon depicted in the story except the simple state of the setting being a fairly average dance marathon of the time resembles June Havoc's experience in any substantial or meaningful way, at least not beyond what experiences would have been shared by a vast number of others who also participated in these marathons. Having a tangential connection to a real life event or experience that would have been shared by countless others doesn't mean any depictions of said event/experience is somehow specifically "telling your story". That would be like me saying "'Matilda' is plagiarism of my life because I was a smart kid who went to a primary school with a mean headteacher too!".

The simple fact is that dance marathons like the one this story is centred around were a legitimately big craze throughout the US beginning in the 1920s and lasted until the late 1930s. During the heyday of the Great Depression it was seen as relatively easy money for those desperate enough to compete, especially those with some kind of performance training or aspiration to that end, and it wasn't unheard of for these marathons to last for weeks. There are even a few cases of such marathons being forcibly shut down due to the deaths of one or more contestants, mostly from exhaustion, and in a 1928 marathon in Seattle there was a relatively well-publicised case where a woman attempted to kill herself in a state of complete mental, physical and emotional hysteria after having ended up in 5th place. Even events like the sprints and "derbies" have basis in the majority of real life dance marathons of the time. Everything in either the book or the movie regarding the marathon has its basis in real life and a lot of people's experiences, not just June Havoc's experiences.